1. Field of the Invention
The present invention relates generally to an optical record disc, and is directed more particularly to an optical record disc of a removing type in which the recording is made in such a manner that its record layer is vaporized or molten away by the heat energy of light or laser beam incident thereon.
2. Description of the Prior Art
As shown in FIG. 1, an example of the prior art optical record disc is formed of a substrate or base plate 101 and a recording layer 102 made of a thin metal film coated on the base plate 101. In this case, concave and convex portions are formed on the base plate 101 in a guide signal region 110, the concave portion is used as a guide signal pit 106. An information pit 104 is formed on the recording layer 102 in an information record region 111 by, for example, a laser beam 103. The above prior art optical record disc is rather low in record sensitivity. That is, in the art the record layer is made of metal such as bismuth Bi, tellurium Te or of similar thin film since it is highest in record sensitivity. Further, in order to improve the record sensitivity and the absorption power for laser beam the recording layer is formed as a multi-layer. In the prior art, however, the record energy of 100 to 30 mJ/cm.sup.2 is required, and for the data record of 10 Mbit/sec a laser light source of the output power more than 20 mW is a practical necessity. A gas laser light source can produce the about output sufficiently, but in this case it is necessary to give information to the laser beam by using an expensive optical modulator. While, if a semiconductor laser which can be directly modulated and has a practical output less than 10 mW is used as the above-mentioned laser light source, it is necessary to expand its radiation area, with the result that the beam spot on the record layer is expanded and hence the energy density thereof is lowered. This will mean that the increase of the power of the semiconductor laser is limited.
As set forth above, since a recording layer of the removing type, which is made of a metal thin film, is insufficient for high speed recording, it has been proposed to use an organic film as the recording layer shown in FIG. 2. In FIG. 2 a base plate 201 is provided with a guide signal hole 206, over which a reflection film 205 is coated, covering the guide signal pit 206. A record layer 202 made of an organic thin film is coated over the reflection film 205. A laser beam 203 which has the guide signal reproduction level intensity within a guide signal region 210, and record level intensity within an information record region 211 to form an information pit 204 is provided.
The above optical record disc is difficult to realize. That is, the organic record layer is made generally in such a manner that a solution containing melted organic substance is coated on a base plate and then its solvent is vaporized. In this case, defects occur when the above solution is charged on the concave pit on the reflection thin film 205 in the guide signal region 210. Defects also occur due to the surface tension of the solution since the surface of the record layer is made flat. The reason is as follows. Since as the absorption ratio of the record layer for the laser beam is high, the record sensitivity of the record layer becomes high, if the guide signal region is formed flat, the intensity of the reflection light i.e. guide signal reproduction light necessary in the guide signal region and the modulation of the light by the guide signal pit are reduced remarkably. As a result, necessary tracking servo can not be operated suitably.